A basic problem in the study of sensory systems is to explain how the structure and development of the brain provide a basis for representation of the environment. The objectives of the research are to define the structural design and the pattern of interrelations of sensory neurons and to relate these histological phenomena to the cytological features of sensory synapses and the factors influencing their formation. Special attention is given to histological analysis of the auditory system with the Golgi and silver- degeneration methods and with electron microscopy. Sufficient details are obtained to correlate with electrophysiological recordings of sensory neurons and with electron microscopic observations of sensory synapses. Histological techniques and experiments are used to reconstruct the sequence of morphological changes in synaptogenesis and to identify the relevant morphogenetic factors. The resulting correlations of structure and function should provide a sound and practical basis for the explanation of the sensory functions of the brain, their development and their disorders.